Equity Transactions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended					9 Months Ended		58 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Jun. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Dec. 31, 2014
Equity [Abstract]
Repurchases of common shares (in shares)						368,468	136,874	943,909
Weighted average purchase price per share (in $ per share)						$ 122.09	$ 191.79	$ 129.95
Total cost of treasury shares acquired						$ 45,000	$ 26,300	$ 122,700
Common stock available to repurchase (in shares)	256,091					256,091		256,091
Cash dividends declared per share (in $ per share)	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.35	$ 0.90
Total cash dividends						$ 12,136	$ 8,333
Dividends payable	$ 4,258					$ 4,258		$ 4,258	$ 4,270
Dividend, to be paid date						Nov. 13, 2015
Dividend, record date						Oct. 30, 2015